Segment Information - Amortization of Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 66	$ 68	$ 14
Commercial Lines
Segment Reporting Information [Line Items]
Amortization of other intangible assets	18	4	1
Property and Casualty, Personal Insurance [Member]
Segment Reporting Information [Line Items]
Amortization of other intangible assets	6	4	4
Group Benefits
Segment Reporting Information [Line Items]
Amortization of other intangible assets	41	60	9
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Amortization of other intangible assets	$ 1	$ 0	$ 0